The variations in Korea-Japan relations have been considerably affected by the U.S. factor. However, no matter how significant roles the U.S. plays in Korea-Japan
relations, an American-centric approach turns our attention away from the simple fact that Korea and Japan themselves are the most responsible for determining their own bilateral relations, and
that the U.S. role in Korea-Japan relations was significant but not determining. Moreover, the U.S. has played ambivalent or contradictory roles which
are sometimes positive and other times negative in advancing the amicable relations between Korea and Japan.

２．The U.S. Role: Important but Not Determinant

U.S. policies could have very complicated and contradictory effects on Korea-Japan relations than we usually imagined. Logically speaking, in the context of
Korea-Japan relations, both the common threats and U.S. engagement in the region could have contradictory effects.

（１）External common threat → Korea-Japan cooperation

The common threats may cause two contradictory outcomes. If external common threats increase, Korea-Japan’s cooperative incentives might
increase. This correlationship is consistent with the Realist logic of balancing. However, although Japan had agreed to form a security pact with the United States, it tried to avoid being deeply
involved in the Cold War politics as much as possible. Despite U.S. opposition, Japan maintained a certain level of economic or private contacts with all its communist neighbors—the Soviet Union,
China, and North Korea—by adopting the so-called policy of seikei bunri (政經分離/“separation of politics and economics”). This policy led South Korea—which was engaged in fierce competition with North Korea—to see Japan as immorally
opportunistic, which often resulted in diplomatic rows in Korea-Japan relations. This kind of Korea-Japan friction should not be attributed to historical animosity; rather, it is an outcome of
‘conflict of interests.’ Different grand strategies of the two states, accompanied by the usual gaps of threat perceptions, could not but create a certain degrees of friction.

If the increase of external threats has the effect of keeping or increasing the level of U.S. engagement, Korea-Japan cooperative incentives
might decrease by increasing Korea-Japan’s free-riding tendency. This correlationship is not consistent with the conventional Realist logic of balancing. Here collective goods logic works. Victor
Cha’s quasi-alliance model focuses on this causality of unintended consequences of the U.S. policies. Instead of utilizing the concept of “free riding,” Cha uses the concept of asymmetric
bilateral abandonment/entrapment structure. It is at least theoretically possible that Japan and the ROK might have developed much stronger security ties if each hadn’t concluded a security
treaty with the United States. Japan and the ROK could afford to “fight” because of the security guarantee from the United States.

U.S. engagement could have a positive effect on Korea-Japan cooperation too. The increased U.S. engagement might motivate the United States to
play a more positive leadership role as a mediator pressuring Japan and South Korea to cooperate. This co-relationship is consistent with the conventional Realist logic of balancing. Here the
focus is on the intended consequence of U.S. policies. Meanwhile, the collective-goods-theory version of the Realist approach like Cha’s quasi-alliance model regards the U.S. engagement as a
negative force that decreases Korea-Japan cooperative incentives.

As Cha points out, Japan and the ROK have symmetric abandonment fears regarding U.S. disengagement from the Northeast Asian region, which
encourages Japanese-ROK cooperation. This correlationship is consistent with collective goods logic, which is adopted by Cha’s quasi-alliance model. However, the causal mechanism of
‘Korea-Japan’s symmetric abandonment fears regarding the United States’ might be actually redundant in explaining Korea-Japan cooperation if we infer that Korea-Japan’s free-riding tendency will
go away when the United States disengages from the region.

In case of the U.S disengagement from the region, besides Korea-Japan symmetric abandonment fears regarding the United States, there could be
different fears. The persistent historical animosity could lead the ROK, facing U.S. disengagement, to have other kinds of asymmetric fears of the resultant collapse of the “double containment”
mechanism not only against common external threats but also against Japan itself. These latter kinds of South Korean fears, by deepening the ROK’s “(adversary) security dilemma”toward Japan,
might increase Korea-Japan friction.

（6）External threat → U.S. engagement → U.S. interventional in Japanese politics → Japan under the rule of conservative political forces → Perception gap on history + historical animosity → No reconciliation between Japan and neighboring
states → Korea-Japan friction

The causal roles of common threats and the U.S. engagement suggest that Realist theory focusing on “material” power can be useful. In the
meantime, the causal roles of perceived common threats and historical animosity indicate that “intersubjective meanings” of the relationship, which
influence the process of identity/interest formation, are also important. However, in reality, material factors and intersubjectives factors may not be inseparable. Regarding Korea-Japan
historical animosity, we can figure out a possible role the United States played right after the war. Some people might wonder why Japan, unlike Germany, has been so reluctant or slow to admit
its war crimes in the past. More accurately speaking, it is the Japanese conservatives that have been reluctant to take responsibility for the wrongdoings inflicted upon the neighboring states
during World War II, which still makes it difficult for Japan to get respect from the neighboring states due to the perception gap on history. The persistent conservative rule in Japanese
domestic politics was in turn helped by the U.S. occupation authority’s “reversal” of the policy direction

from
demilitarization and democratization of Japan towards strengthening of Japan as a Cold War bulwark against the communist states. Accordingly, the United States helped the conservative political
forces—Liberal Democratic Party—to rule Japan for a long time. In short, the U.S. intervention in Northeast Asian regional politics, though originally intended to promote security cooperation
among the anticommunist states, probably has had a long-term negative effect on the process of reconciliation between Japan and other states, including South Korea.

３．Conclusion

We
cannot categorically say that the U.S. has been of help in advancing Korea-Japan relations. Moreover, we should not strengthen ‘exclusive’ U.S.-Japan-Korea security ties against others too
tightly but develop the more ‘inclusive’ regional security ties in Northeast Asia, including China, Russia or North Korea. Which prevails will determine the destiny of the new world order.